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From the 2015 UPC Illustrated Training Manual

404.0 Overflows.
404.1 General. Where a fixture is provided with an overflow, the waste shall be so arranged that the standing water in the fixture shall not rise in the overflow where the stopper is closed or remain in the overflow where the fixture is empty. The overflow pipe from a fixture shall be connected on the house or inlet side of the fixture trap, except that overflow on flush tanks shall be permitted to discharge into the water closets or urinals served by them, but it shall be unlawful to connect such overflows with any other part of the drainage system.

Not all plumbing fixtures are required to have an overflow. For example, the applicable standards for plastic, enameled cast iron, enameled steel, and ceramic plumbing fixtures do not require overflows for lavatories, sinks and bidets. However, the standards require that bathtubs and whirlpools meet the minimum dimensions shown in the standard, which includes an overflow. When an overflow is provided, whether mandatory or optional, the fixture waste shall be so arranged that the standing water in the fixture cannot rise in the overflow when the stopper is closed or remain in the overflow when the fixture is empty. The only time water is permitted in the fixture overflow is when the fixture is filled to the point of overflow.

The overflow pipe from a fixture must connect only on the house or inlet side of a fixture trap with the exception of the overflow on a flush tank. The flush tank may discharge into the water closet or urinal it serves. An overflow is not permitted to bypass the trap of the fixture it serves. It may either be an integral part of the fixture or a separate fitting (see Figure 404.1).

When waste and overflow fittings are used and are concealed, as in most bathtub installations, they must be constructed from materials approved for drainage pipe. When they are exposed or accessible, they may be of seamless drawn brass, not less than No. 20 B & S gauge (0.032 in.) (0.8 mm) in thickness, or other approved material as per Section 701.4.

The 2015 Uniform Plumbing Code Illustrated Training Manual eBook is available for purchase here.